1. Field of the Invention
The present invention relates to communication systems, and more particularly, cellular communication systems.
2. Background of the Related Art
The use of code division multiple access (CDMA) modulation techniques is one of several techniques for facilitating communications in which a large number of systems are present. FIG. 1 generally illustrates a system 10, which uses CDMA modulation techniques in communication between user equipment (UE) 12a and 12b, each UE including a cellular telephone, and base stations (BTS) 14a and 14b. A base station controller (BSC) 16 typically includes an interface and processing circuitry for providing system control to the BTS 14a, 14b. The BSC 16 controls the routing of telephone calls from the public switched telephone network (PSTN) to the appropriate BTS for transmission to the appropriate UE. The BSC 16 also controls the routing of calls from the UEs, via at least one BTS to the PSTN. The BSC 16 may direct calls between UEs via the appropriate BTS since UEs do not typically communicate directly with one another. The BSC 16 may be coupled to the BTS 14a and 14b by various means including dedicated telephone lines, optical fiber links or by microwave communication links.
The arrows 13a-13d define the possible communication links between the BTS 14a and UEs 12a and 12b. The arrows 15a-15d define the possible communication links between the BTS 14ba and UEs 12a and 12b. In the reverse channel or uplink (i.e., from UE to BTS), the UE signals is received by BTS 14a and/or BTS 14b, which, after demodulation and combining, pass the signal forward to the combining point, typically to the BSC 16. In the forward channel or downlink (i.e., from BTS to UE), the BTS signals are received by UE 12a and/or UE 12b. The above system is described in U.S. Pat. Nos. 5,101,501; 5,103,459; 5,109,390; and 5,416,797, whose entire disclosure is hereby incorporated by reference therein.
A radio channel is a generally hostile medium in nature. It is rather difficult to predict its behavior. Traditionally, the radio channels are modeled in a statistical way using real propagation measurement data. In general, the signal fading in a radio environment can be decomposed into a large-scale path loss component together with a medium-scale slow varying component having a log-normal distribution, and a small-scale fast varying component with a Rician or Rayleigh distribution, depending on the presence or absence of the line-of-sight (LOS) situation between the transmitter and the receiver.
FIG. 2 illustrates these three different propagation phenomena. An extreme variation in the transmission path between the transmitter and receiver can be found, ranging from direct LOS to severely obstructed paths due to buildings, mountains, or foliage. The phenomenon of decreasing received power with distance due to reflection, diffraction around structures, and refraction is known as path loss.
As shown, the transmitted signal is reflected by many obstacles between a transmitter and a receiver, thus creating a multipath channel. Due to the interference among many multipaths with different time delays, the received signal suffers from frequency selective multipath fading. For example, when the 2 GHz carrier frequency band is used and a car having a UE is travelling at a speed of 100 km/h, the maximum Doppler frequency of fading is 185 Hz. While coherent detection can be used to increase link capacity, under such fast fading, the channel estimation for coherent detection is generally very difficult to achieve. Because of fading channels, it is hard to obtain a phase reference for the coherent detection of data modulated signal. Therefore, it is beneficial to have a separate pilot channel.
Typically, a channel estimate for coherent detection is obtained from a common pilot channel. However, a common pilot channel transmitted with an omnidirectional antenna experiences a different radio channel than a traffic channel signal transmitted through a narrow beam. It has been noticed that common control channels are often problematic in the downlink when adaptive antennas are used. The problem can be circumvented by user dedicated pilot symbols, which are used as a reference signal for the channel estimation. The dedicated pilot symbols can either be time or code multiplexed.
FIG. 3 depicts a block diagram of a transmitter and a receiver for time multiplexed pilot symbols for an improved channel estimation method that works satisfactorily under slow-to-fast fading environments. Known pilot symbols are periodically multiplexed with the sequence of the transmitted data. The pilot symbols and data symbols following pilot symbols constitute a slot, as shown in FIG. 3.
Further, in a DS-CDMA transmitter, the information signal is modulated by a spreading code, and in the receiver, it is correlated with a replica of the same code. Thus, low cross-correlation between the desired and interfering users is important to suppress the multiple access interference. Good autocorrelation properties are required for reliable initial synchronization, since large sidelobes of the autocorrelation function may lead to erroneous code synchronization decisions. Furthermore, good autocorrelation properties are important to reliably separate the multipath components.
Since the autocorrelation function of a spreading code should resemble, as much as possible, the autocorrelation function of white Gaussian noise, the DS code sequences are also called pseudo-noise (PN) sequences. The autocorrelation and cross-correlation functions are connected in such a way that it is not possible to achieve good autocorrelation and cross-correlation values simultaneously. This can be intuitively explained by noting that having good autocorrelation properties is also an indication of good randomness of a sequence. Random codes exhibit worse cross-correlation properties than deterministic codes.
Such mobile communication system has gone through different stages of evolution, and various countries used different standards. First generation mobile systems in the 1980s used analog transmission for speech services. Advanced Mobile Phone Service (AMPS) in the United States, Total Access Communication System (TACS) in the United Kingdom, Nordic Mobile Telephones (NMT) in Scandinavia, Nippon Telephone and Telegraph (NTT) in Japan, etc., belonged to the first generation.
Second generation systems using digital transmission were introduced in the late 1980s. They offer higher spectrum efficiency, better data services, and more advanced roaming than the first generation systems. Global System for Mobile Communications (GSM) in Europe, Personal Digital Cellular (PDC) in Japan, and IS-95 in the United States belonged to the second generation.
Recently, third generation mobile radio networks have been under intense research and discussion and will emerge around the year 2000. In the International Telecommunication Union (ITU), the third generation networks are called International Mobile Telecommunications-2000 (IMT-2000) and in Europe, Universal Mobile Telecommunication System (UMTS). IMT-2000 will provide a multitude of services, including multimedia and high bit rate packet data.
Wideband CDMA has emerged as the mainstream air interface solution for the third generation networks. Wideband CDMA systems are currently being standardized by the European Telecommunications Standards Institute (ETSI) of Europe, the Association for Radio Industry and Business (ARIB) of Japan, the TIA Engineering Committees TR45 and TR46 and the T1 committee T1P1 of the United States, and the Telecommunication Technology Association TTA I and TTA II (renamed Global CDMA I and II, respectively) in Korea. The above description and a background of various systems can be found in WIDEBAND CDMA FOR THIRD GENERATION MOBILE COMMUNICATIONS by T. Ojanpera et al, published 1998, by Artech House Publishers, whose entire disclosure is hereby incorporated by reference therein.
Recently, ARIB in Japan, ETSI in Europe, T1 in U.S.A., and TTA in Korea have mapped out a third generation mobile communication system based on a core network and radio access technique of an existing global system for mobile communications (GSM) to provide various services including multimedia, such as audio, video and data. They have agreed to a partnership study for the presentation of a technical specification on the evolved next generation mobile communication system and named a project for the partnership study as a third generation partnership project (3GPP).
The 3GPP is classified into three part technical studies. The first part is a 3GPP system structure and service capability based on the 3GPP specification. The second part is a study of a universal terrestrial radio access network (UTRAN), which is a radio access network (RAN) applying wideband CDMA technique based on a frequency division duplex (FDD) mode, and a TD-CDMA technique based on a time division duplex (TDD) mode. The third part is a study of a core network evolved from a second generation GSM, which has third generation networking capabilities, such as mobility management and global roaming.
Among the technical studies of the 3GPP, the UTRAN study defines and specifies the transport and physical channels. This technical specification, TS S1.11 v 1.1.0, was distributed on March of 1999, whose entire disclosure is hereby incorporated by reference therein. The physical channel includes the dedicated physical channels DPCHs) used in the uplink and downlink. Each DPCH is generally provided with three layers, e.g., superframes, radio frames and timeslots. As specified in the 3GPP radio access network (RAN) standard, a superframe has a maximum frame unit of 720 ms period. In view of the system frame numbers, one superframe is composed of seventy-two radio frames. Each radio frame has a period of 10 ms, and a radio frame includes sixteen timeslots, each of which includes fields with corresponding information bits based on the DPCH.
FIG. 4 illustrates a frame structure of an uplink DPCH based on the 3GPP RAN standard. The uplink DPCH is provided with two types of channels, e.g., a dedicated physical data channel (DPDCH) and a dedicated physical control channel (DPCCH). The uplink DPDCH is adapted to transport the dedicated data and the uplink DPCCH is adapted to transport the control information.
The uplink DPCCH for the transport of the control information includes various fields such as a pilot field 21 of Npilot bits, a transmit power-control (TPC) field 22 of NTPC bits, a feedback information (FBI) field 23 of NFBI bits and an optional transport-combination indicator (TFCI) field 24 of NTFCI bits. The pilot field 21 includes pilot bits Npilot for supporting channel estimation for coherent detection. The TFCI field 4 supports the simultaneous provision of a plurality of services by the system. The absence of the TFCI field 4 in the uplink DPCCH signifies that the associated service is a fixed rate service. The parameter k determines the number of bits per uplink DPDCH/DPCCH slot. It is related to the spreading factor SF of the physical channel as SF=256/2k. The spreading factor SF may thus range from 256 down to 4.
FIG. 5 is a table showing various information of the uplink DPCCH, wherein channel bit and symbol rates are those just prior to spreading. (At the time of this technical specification, the exact number of bits of the different uplink DPCCH fields of FIG. 4 (Npilot, NTPC, NFBI, and NTFCI) was not determined.)
FIG. 6 is a table illustrating pilot bit patterns of the uplink DPCCH, and more particularly, 6-bit and 8-bit pilot bit patterns for each slot. In FIG. 6, the non-shaded sequence is used for channel estimation, and shaded sequence can be used as frame synchronization words or sequences. The pilot bits other than frame synchronization word, e.g., channel estimation word, have a value of 1.
For example, in the case where each slot includes six pilot bits Npilot=6, the sequences formed by slot #1 to slot #16 at bit #1, at bit #2, at bit #4, and at bit #5 are used as the frame synchronization words. In the case where each slot is composed of eight pilot bits (Npilot=8), the sequences at bit #1, at bit #3, at bit #5, and at bit #7 are used as the frame synchronization words. In the case where the pilot bits of each sequences slot are either 6 or 8 in number, a total of four is used as the frame synchronization word. As a result, because one radio frame is provided with sixteen timeslots, the number of pilot bits used as the frame synchronization word is 64 bits per frame.
FIG. 7 shows a spreading/scrambling arrangement for the uplink DPCH based on the 3GPP RAN standard. The arrangement of FIG. 7 is provided for the execution of a quadrature phase shift keying (QPSK) operation where the uplink DPDCH and DPCCH are mapped into I and Q channel branches, respectively.
The spreading is an operation for switching all symbols through the respective channel branches to a plurality of chips. The I and Q channel branches are spread respectively at chip rates based on two different orthogonal variable spreading factors (OVSFs), or channelizing codes CD and CC. The OVSF represents the number of chips per symbol on each channel branch. The spread of two channel branches are summed and then complex-scrambled by a specific complex scrambling code Cscramb. The complex-scrambled result is separated into real and imaginary and then transmitted after being placed on respective carriers.
FIG. 8 illustrates a frame structure of a downlink DPCH based on the 3GPP RAN standard. The number of pilot bits (or symbols) in the uplink DPCH is 6 or 8 because the uplink DPCH is activated at a fixed rate of 16 Kbps. However, since the downlink DPCH is activated at a variable rate, it has pilot symbol patterns illustrated in FIG. 9.
With reference to FIG. 8, similar to the uplink DPCH, the downlink DPCH is provided with two types of channels, e.g., a dedicated physical data channel (DPDCH) and a dedicated physical control channel (DPCCH). In the downlink DPCH, the downlink DPDCH is adapted to transport the dedicated data and the downlink DPCCH is adapted to transport the control information. The downlink DPCCH for transporting the control information is composed of various fields such as a pilot field 27, TPC field 26 and TFCI field 25. The pilot field 27 includes pilot symbols for supporting the channel estimation for coherent detection.
FIG. 9 is a table illustrating pilot symbol patterns contained in the downlink DPCCH, which are classified according to different symbol rates of the downlink DPCCH. For example, in the case where the symbol rate is 16, 32, 64 or 128 Kbps, each slot includes four pilot symbols for an I channel branch and four pilot symbols for a Q channel branch, totaling eight pilot symbols.
In FIG. 9, the non-shaded sequence is used for channel estimation and shaded sequences can be used as frame synchronization words. The remaining pilot symbols other than the frame synchronization word (e.g., channel estimation) have a value of 11. For example, in the case where the symbol rate is 16, 32, 64 or 128 Kbps, the sequences, formed by pilot symbols from slot #1 to slot #16, at symbol #1 and at symbol #3 are used as the frame synchronization words. Accordingly, because the number of pilot symbols used as the frame synchronization words is 4 per slot, 64 pilot symbols are used in each radio frame.
FIG. 10 illustrates a spreading/scrambling arrangement for the downlink DPCH based on the 3GPP RAN standard. The arrangement of FIG. 10 is provided for the spreading and scrambling of the downlink DPCH and a common control physical channel (CCPCH). A QPSK operation is performed with respect to a pair of symbols of the two channels in such a manner that they are serial-to-parallel converted and then mapped into I and Q channel branches, respectively.
The I and Q channel branches are spread respectively at chip rates based on two equal channelizing codes Cch. The spread of the two channel branches are summed and then complex-scrambled by a specific complex scrambling code Cscramb. The complex-scrambled result is separated into real and imaginary and then transmitted, after being placed on respective carriers. Noticeably, the same scrambling code is used for all physical channels in one cell, whereas different channelizing codes are used for different physical channels. Data and various control information are transported to a receiver through the uplink and downlink DPCHs subjected to the above-mentioned spreading and scrambling.
The TS S1.11 v1.1.0 specification also specified a primary common control physical channel (PCCPCH), which is a fixed rate downlink physical channel used to carry the broadcast channel (BCH), and a secondary common control physical channel (SCCPCH) used to carry the forward access channel (FACH) and the paging channel (PCH) at a constant rate. FIGS. 11A and 11B illustrate the frame structure of PCCPCH and SCCPCH, each having a pilot field. The TS S1.11 v1.1.0 specification recommended the pilot patterns for the PCCPCH and SCCPCH. Further, the TS S1.11 v1.1.0 specification recommended the pilot pattern of the DPCH channel for the diversity antenna using open loop antenna diversity based on space time block coding based transmit diversity (STTD) and diversity antenna pilot patterns for PCCPCH and SCCPCH. Those patterns can be found in the TS S1.11 v1.1.0 specification, and detailed description is being omitted.
For frame synchronization, an autocorrelation function must be performed on the basis of the pilot pattern sequence. In the pilot sequence design, finding an autocorrelation of a sequence with the lowest out-of-phase coefficient is important to decrease the probability of false alarm regarding the synchronization. A false alarm is determined when a peak is detected when there should not be a peak detection.
Optimally, the result of the autocorrelation for a frame with a sequence at a prescribed pilot bit should have same maximum values at zero and middle time shifts of one correlation period, which are different in polarity, and the remaining sidelobes at time shifts other than zero and middle should have a value of zero. However, the various pilot patterns recommended in the TS S1.11 v1.1.0 do not meet this requirement, both in the uplink and downlink.
In an article entitled xe2x80x9cSynchronization Sequence Design with Double Thresholds for Digital Cellular Telephonexe2x80x9d by Young Joon Song et al. (Aug. 18-20, 1998), the present inventor being a co-author, the article describes a correlator circuit for GSM codes where the out-of-phase coefficients are all zero except one exception at zero and middle shift having a first peak and a second peak, where the first and second peaks are opposite in polarity, but the peaks are not equal to one another. Further, the article describes lowest out-of-phase coefficients of +4 and xe2x88x924. However, the article does not provide how such sequences and autocorrelation can be used to achieve the above described optimal results, and the article does not provide sufficient disclosure that the sequences achieve or can achieve the lowest autocorrelation sidelobes.
As described above, the pilot patterns used as frame synchronization words or sequences do not achieve the optimal results. Further, the background pilot patterns do not rapidly and accurately perform the frame synchronization. Moreover, the above pilot patterns and frame synchronization sequences do not provide optimal cross-correlation and autocorrelation. Additionally, neither the TS specification nor the article provides a solution of the use of the pilot patterns for slot-by-slot double check frame synchronization scheme, and neither discloses the use of the frame synchronization sequence for channel estimation.
An object of the present invention is to obviate at least the problems and disadvantages of the related art.
An object of the present invention is to provide frame synchronization words resulting in optimal autocorrelation results.
A further object of the present invention is to eliminate or prevent sidelobes.
A further object of the present invention is to provide maximum values at zero and middle time shifts.
Another object of the present invention is to provide a synchronization word for at least one of rapid and accurate frame synchronization.
Another object of the present invention is to provide a slot-by-slot double check frame synchronization scheme.
Still another object of the present invention is to provide a frame synchronization word which can be used for channel estimation.
Still another object of the present invention is to provide good cross-correlation and autocorrelation simultaneously.
The present invention can be achieved in a whole or in parts by a method for synchronizing a frame using an optimal pilot symbol, comprising the steps of: (1) receiving a pilot symbol of each slot in the frame through respective physical channels on a communication link; (2) correlating a received position of each of the pilot symbols to a corresponding pilot sequence; (3) combining and summing more than one results of the correlations, and deriving a final result from the correlations in which sidelobes from the results of the correlations are offset; and (4) synchronizing the frame using the final result.
The pilot symbols are combined into each of the pilot sequences such that the final result of the correlations shows sidelobes with 0xe2x80x3 values excluding particular positions of correlation periods. The particular positions are starting points (x=0) of the correlation periods (x) and points of x/an integer. The pilot symbol is a combination of pilot symbols in a form of (a,/a). The pilot sequence provides least correlation resultants at positions excluding the starting points and half of the starting points in the correlation periods. The pilot symbols excluding the pilot symbols used in the correlation is used in a channel estimation for detecting coherent. The pilot symbol of each slot in the frame is transmitted, with the pilot symbol contained in a pilot field of an exclusive physical control channel among respective exclusive channels on the communication link. The pilot sequences different from each other on an up communication link are used in the correlation according to values of bits included in a pilot field of an exclusive physical control channel. The pilot sequences different from each other on a down communication link are used in the correlation according to a symbol rate of an exclusive physical control channel.
The present invention can be also achieved in a whole or in parts by a method for synchronizing a frame using an optimal pilot symbol, comprising the steps of: (1) receiving a pilot symbol of each slot in the frame through respective physical channels on a communication link; (2) correlating a received position of each of the pilot symbols to a corresponding pilot sequence; (3) combining and summing more than one results of the correlations, and deriving a final result from the correlations in which sidelobes from the results of the correlations have minimum values and the results of the correlations at starting points and middle points of correlation periods have maximum values with different polarity; and (4) synchronizing the frame using the final result.
The present invention can be achieved in a whole or in parts by a method of eliminating sidelobes in a communication channel between a base station and a mobile station, comprising the steps of: generating control signals and data signals within the communication channel, the control signals having a first sequence of L-bits and a second sequence of L-bits; generating a first set of prescribed values based on the first sequence, which has a first prescribed relationship with the first set of prescribed values; generating a second set of prescribed values based on the second sequence, which has a second prescribed relationship with the second set of prescribed values; and combining the first and second sets of prescribed values.
The present invention can be achieved in a whole or in parts by a method of establishing a communication channel, the method comprising the steps of: generating a plurality of frames; generating a L-number of slots for each frame, each slot having a pilot signal of N-bits and a corresponding bit in each slot forming a word of L-sequence of pilot bits such that there is N number of words, wherein the number of bit values of two pilot bits which are the same between two adjacent words from 1 to L slots minus the number of bit values of two pilot bits which are different between the two adjacent words from 1 to L is zero or a prescribed number close to zero.
The present invention can be achieved in a whole or in parts by a method of establishing a communication channel having at least one of frame synchronization and channel estimation, the method comprising the steps of: generating a plurality of frames; generating a L-number of slots for each frame, each slot having a pilot signal of N-bits and a corresponding bit in each slot forming a word of L-sequence of pilot bits such that there is N number of words, wherein the words have at least one of the following characteristics: cross-correlation between two adjacent sequences used for frame synchronization is zero at zero time shift, or cross-correlation between a word used for frame synchronization and a word used for channel estimation is zero at all time shifts.
The present invention can be achieved in a whole or in parts by a method of reducing sidelobes for frame synchronization, comprising the steps of: generating a plurality of frame synchronization words, each frame synchronization word having a plurality of bits; performing autocorrelation functions on a pair of frame synchronization words to generate a pair of prescribed value sets; and combining the pair of prescribed value sets such that two peak values equal in magnitude and opposite in polarity are achieved at zero and middle time shifts.
The present invention can be achieved in a whole or in parts by a method of generating pilot signals of a prescribed pattern within a frame having L-number of slots, comprising the steps of: generating N-number of pilot bits for each slot; and forming N-number of words of L-bit based on above step, wherein a prescribed number of words is used for frame synchronization words and each frame synchronization word has a first prescribed number b0 of bit values of xe2x80x9c0xe2x80x9d and a second prescribed number b1 of bit values of xe2x80x9c1xe2x80x9d, such that b1-b0 is equal to zero or a number close to zero.
The present invention can be achieved in a whole or in parts by a communication link between a user equipment and a base station comprising a plurality of layers, wherein one of the layers is a physical layer for establishing communication between the user equipment and the base station and the physical layer has at least one of data and control information, one of the control information being a pilot field of N-bits transmitted for L-number of slots such that N-number of words of L-bit are formed, wherein cross-correlation between two adjacent words used for frame synchronization is zero at zero time shift or cross-correlation between a word used for frame synchronization and a word used for channel estimation is zero at all time shifts.
The present invention can be achieved in a whole or in parts by a correlator circuit for at least one of a user equipment and a base station, comprising: a plurality of latch circuits, each latch circuit latching a word formed by a pilot bit from a plurality of slots; a plurality of correlators, each correlator coupled to a corresponding latch circuit and correlating the word to a set of prescribed values; and a combiner that combines the set from each correlator such that maximum peak values of equal in magnitude and opposite in polarity are formed at zero and middle time shifts.
The present invention can be achieved in a whole or in parts by a communication device comprising: means for transmitting at least one of data and control information; means for receiving at least one of data and control information, wherein the receiving means includes: a plurality of latch circuits, each latch circuit latching a word formed by a pilot bit from a plurality of slots; a plurality of correlators, each correlator coupled to a corresponding latch circuit and correlating the word to a set of prescribed values; a plurality of buffers, each buffer coupled to a corresponding correlator to store the set of prescribed values; and a combiner that combines the set from each buffer such that maximum peaks of equal in magnitude and opposite in polarity are formed at zero and middle time shifts.
Additional advantages, objects, and features of the invention will be set forth in part in the description which follows and in part will become apparent to those having ordinary skill in the art upon examination of the following or may be learned from practice of the invention. The objects and advantages of the invention may be realized and attained as particularly pointed out in the appended claims.